Bottom Line:
Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices.It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions.Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

ABSTRACTIntensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices. It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions. Here, using NiO as an example, we show that rationally adjusting the stoichiometry and the associated defect characteristics enables controlled room temperature conversions between two distinct RS modes, i.e., nonvolatile memory switching and volatile threshold switching, within a single device. Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

f3: Determination of compositions in various RS states.(a) SIMS profiles for different RS states. In comparison to the stoichiometric pristine state, the estimated average concentrations of oxygen vacancies are 7.7% and 6.3% for the memory ON and OFF states, respectively. The average concentration of nickel vacancies is 1.5% for the threshold OFF state. (b) PF emission fittings to the pristine and the threshold OFF states. The derived dielectric constant of the threshold OFF state is larger than that of pristine state, in line with the SIMS result that the O/Ni ratio is higher in the threshold OFF state. (c) Schematic of the Ni-rich metallic filament in NiO whose formation and rupture is the result of dynamic competition between the oxygen-gradient-induced drift and the field-induced diffusion motions of the oxygen ions.

Mentions:
To further analyze the stoichiometry of devices showing different switching behaviors, we performed the time-of-flight secondary ion mass spectroscopy (TOF-SIMS) measurements after the devices were switched to different states and compared the data to the standard NiO sample. The SIMS profiles of normalized O/Ni ratios for different switching states are shown in Figure 3a. In general, the O/Ni ratio is higher near the interface of NiO/ITO because the ITO bottom electrode serves as a reservoir of oxygen ions. All depth-resolved compositional profiles exhibit notable fluctuations; nevertheless, in average, the O/Ni ratio is the highest in the threshold OFF state, and the lowest in the memory ON state. This is consistent with the previous reports where the threshold switching was observed in Ni-deficient NiO films14. Since in NiO the formation energies of vacancies are much lower than those of interstitials38, the off-stoichiometry in different switching states can be attributed to the presence of either oxygen or nickel vacancies. The estimated average concentrations of oxygen vacancies in the memory OFF and ON state are 6.3 at.% and 7.7 at.%, respectively; this off-stoichiometry could be related to the existence of Ni-rich metallic nanofilaments which were suggested to be essential for RS in NiO37. On the other hand, the average concentration of nickel vacancies in the threshold OFF state is approximately 1.5 at.%.

f3: Determination of compositions in various RS states.(a) SIMS profiles for different RS states. In comparison to the stoichiometric pristine state, the estimated average concentrations of oxygen vacancies are 7.7% and 6.3% for the memory ON and OFF states, respectively. The average concentration of nickel vacancies is 1.5% for the threshold OFF state. (b) PF emission fittings to the pristine and the threshold OFF states. The derived dielectric constant of the threshold OFF state is larger than that of pristine state, in line with the SIMS result that the O/Ni ratio is higher in the threshold OFF state. (c) Schematic of the Ni-rich metallic filament in NiO whose formation and rupture is the result of dynamic competition between the oxygen-gradient-induced drift and the field-induced diffusion motions of the oxygen ions.

Mentions:
To further analyze the stoichiometry of devices showing different switching behaviors, we performed the time-of-flight secondary ion mass spectroscopy (TOF-SIMS) measurements after the devices were switched to different states and compared the data to the standard NiO sample. The SIMS profiles of normalized O/Ni ratios for different switching states are shown in Figure 3a. In general, the O/Ni ratio is higher near the interface of NiO/ITO because the ITO bottom electrode serves as a reservoir of oxygen ions. All depth-resolved compositional profiles exhibit notable fluctuations; nevertheless, in average, the O/Ni ratio is the highest in the threshold OFF state, and the lowest in the memory ON state. This is consistent with the previous reports where the threshold switching was observed in Ni-deficient NiO films14. Since in NiO the formation energies of vacancies are much lower than those of interstitials38, the off-stoichiometry in different switching states can be attributed to the presence of either oxygen or nickel vacancies. The estimated average concentrations of oxygen vacancies in the memory OFF and ON state are 6.3 at.% and 7.7 at.%, respectively; this off-stoichiometry could be related to the existence of Ni-rich metallic nanofilaments which were suggested to be essential for RS in NiO37. On the other hand, the average concentration of nickel vacancies in the threshold OFF state is approximately 1.5 at.%.

Bottom Line:
Intensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices.It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions.Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.

ABSTRACTIntensive investigations have been launched worldwide on the resistive switching (RS) phenomena in transition metal oxides due to both fascinating science and potential applications in next generation nonvolatile resistive random access memory (RRAM) devices. It is noteworthy that most of these oxides are strongly correlated electron systems, and their electronic properties are critically affected by the electron-electron interactions. Here, using NiO as an example, we show that rationally adjusting the stoichiometry and the associated defect characteristics enables controlled room temperature conversions between two distinct RS modes, i.e., nonvolatile memory switching and volatile threshold switching, within a single device. Moreover, from first-principles calculations and x-ray absorption spectroscopy studies, we found that the strong electron correlations and the exchange interactions between Ni and O orbitals play deterministic roles in the RS operations.